The present invention relates to a method and apparatus for determining the stability margin, with respect to a possible self-oscillation, in a radio frequency repeater operating with a predetermined delay between an input and an output and having a feedback path between said output and said input. The invention also relates to a repeater system provided with such an apparatus.
Primarily, the invention concerns repeaters for use in cellular telecommunication systems, including two antennas and two sets amplifier chains for amplifying a signal from a base station (down-link) and a signal from a cellular mobile telephone (up-link), respectively. In such a system, the repeater recieves, amplifies and retransmits signals between the base station and the mobile telephone in both directions. However, the invention can be applied also to repeaters with only one signal path.
More particularly, the invention deals with the problem of ensuring that the repeater does not reach a state of instability because of self-oscillation, which may happen when the gain becomes too high. Because of the inevitable feed back between the output and the input, the too strongly amplified output signal will be transferred back to the input and will be amplified over again so as to cause a self-oscillation in the system.
The problem of preventing self-oscillation and securing a sufficient margin to such an instability in repeaters is well-known to those skilled in the art. Normally, in order to solve this problem, the input or output signal level (or power level) is monitored during operation of the repeater.
One way, see U.S. Pat. No. 5,095,528 (Leslie), is to continuously measure the average output power of the repeater and to determine whether this power level exceeds a predetermined threshold value, in which case the amplifier gain of the repeater is drastically reduced so as to restore a stable state of operation.
Another way, which is especially useful in GSM systems operating with time slots, see WO 97/33381 (Allgon), is to measure the time proportion during which the power level of an input signal exceeds a certain threshold value, whereupon the amplifier gain is reduced in case this time proportion is relatively large, such as 90% or even larger. This method may be combined with a test whether the difference between the maximum and minimum values of the input signal level is less than a given value.
However, in both these known methods, like in all other methods being used in repeaters today, it is necessary to adjust the gain rather drastically in response to a detected rise of the signal level. Basically, this is because the criteria being used are met only when a state of instability is reached. Thus, in the known systems, it is not possible to obtain a quantified value of the stability margin as such. Therefore, it is difficult to provide a smooth control of the amplifier gain of the repeater.
Against this background, a main object of the present invention is to provide a method and an apparatus for determining the stability margin, with respect to a possible self-oscillation, in a repeater of the kind discussed above. Another object is to provide a method which would enable a smooth control of the repeater without drastic changes of the amplifier gain. Accordingly, it should be possible to establish a quantified value of the stability margin as such.
These and other objects are achieved, according to the present invention, by taking two basic steps, viz.
establishing the amplification of the repeater as a function of the frequency in a frequency band (which should be relatively wide), and
observing the magnitude of wave-like variations in the established amplication as a function of the frequency, this magnitude constituting a measure of the stability margin such that an increasing magnitude corresponds to a decreasing stability margin.
In this way, it is possible to obtain a measure of the stability margin even when the system operates normally and is still stable. Consequently, any control measures can be taken already long before the system approaches a state of instability, and the control can therefore be performed smoothly, i.e. with relatively small adjustments of the amplifier gain of the system.
When using an appropriate control algorithm, the system can be retained in a stable state without self-oscillation.
The method can be carried out by means of a measurement receiver connected to the input and/or the output of the repeater by means of a directional coupler, preferably via a switch alternating between the input and the output.
Advantageously, in order to establish the amplification or frequency response in the particular frequency band, provided that the spectral properties of the input signal are known, it is sufficient to measure the output signal as a function of the frequency. Then, the frequency response and the stability margin can be calculated.
The invention will be explained further below with reference to the appended drawings illustrating a repeater and a measurement receiver for determining the stability margin in accordance with the invention.